Hepatocellular carcinoma (HCC) is the sixth most common cancer worldwide and third highest cause of cancer-related deaths. Although the development of HCC has been linked to exposure to various toxins or infectious agents, the majority of HCC cases are associated with chronic hepatitis B virus (HBV) infections. Worldwide, there are over 350 million people chronically infected with HBV, and approximately 25 percent of these individuals will eventually develop HCC. The high incidence of HBV infections, the high mortality rate of individuals with HCC, the increasing occurrence of HCC, and the strong association between chronic HBV infections and HCC development indicates that investigating the mechanisms underlying the development of HBV-associated HCC is a critically important area of investigation. While not entirely understood, these underlying mechanisms are thought to involve the immune-mediated destruction of HBV-infected hepatocytes and compensatory liver regeneration as well as activities of the multifunctional HBV X protein (HBx). HBx has been shown to be required for HBV replication and is thought to play at least a co-factor role in HCC development. Ca2+ signaling influences almost every cellular process and has been shown to play a critical role in many aspects of cancer development; the Ca2+-binding protein calmodulin (CaM) is thought to be a major contributor to cancer development and is required for cell proliferation. Interestingly, HBx regulation of Ca2+ signals is thought to act as an upstream initiating event for many other HBx effects, and Ca2+ has been shown to be required for HBx regulation of cell proliferation, apoptosis and HBV replication. We hypothesize that HBx modulates store-operated Ca2+ (SOC) entry (SOCE) to increase cytosolic Ca2+ levels and stimulate HBV replication, while consequently de-regulating normal hepatocyte Ca2+ signaling to increase CaM activation and CaM-dependent modulation of cell proliferation. The goal of this application is to define the effect of HBx on SOCE in cultured primary rodent hepatocytes, a biologically relevant model system, and how this regulation affects HBV replication, CaM signaling, and CaM-mediated cell proliferation. In Aim 1, we will investigate HBx regulation of SOCE using kinetic Ca2+ studies. We will also determine the impact of HBx on expression levels of SOC channel components and how this affects HBV replication. In Aim 2, we will investigate additional consequences of HBx regulation of Ca2+ by analyzing the impact of HBx on CaM signaling and CaM regulation of cell proliferation. In Aim 3, we will address the significance of HBx regulation of SOCE and CaM signaling for HCC development by utilizing an HBx-transgenic mouse liver cancer model; we will disrupt SOCE and CaM in these mice and determine the impact on HCC development. Overall, we expect that our studies will identify mechanisms by which HBx alters hepatocyte physiology to promote HBV replication and HCC development. Our studies may also identify new therapeutic targets for blocking HBx activities and HBV replication and the development of HBV-associated HCC.